I. Field of the Invention
The invention relates to an apparatus for measuring strain in a solid object, and concerns apparatus arranged to use the deflection of a light guiding structure as a strain responsive element for determining the amount of applied stress to be measured.
II. Prior Art
Microbending losses in optical fibers have been thoroughly studied by theoreticians interested in the application of electromagnetic theory to a new guiding medium, and experimentally by fiber manufacturers who wish to avoid the adverse effects of bending on fiber characteristics.
Present theory is largely based on the early work of Marcatili in his publications in the Bell System Technical Journal, 1969, Volume 48, page 2103 and 2161, who investigated in a general way the effect of perturbations on light guiding structures. Possible loss mechanisms for light travelling in a bent guide are:
(1) Radiative loss. Some of the light traversing a bend may be radiated sideways out of the guide. The radiation attenuation coefficient (.alpha..sub.r) can be calculated from EQU .alpha.=C.sub.1 exp (-C.sub.2 r)
where
R=RADIUS OF CURVATURE OF THE BEND
C.sub.1, C.sub.2 =constants independent of r.
(2) Mode coupling loss. Bending a guide creates higher order modes which may be attenuated by a lossy cladding.
(3) Mode conversion loss. This occurs for a straight guide mode entering or leaving a curved region.
A more simple, empirical description of microbending loss behaviour is provided by Zeidler in his publication in Optics Communications, 1976, volume 18, page 553. In an experimental system, the effect of periodic perturbations of an optical fiber was studied. The attenuation (.alpha.) of the light (in dB/m in the fiber was described by the following empirical power law) EQU .alpha.=b.sub.1 .multidot..DELTA..sup.b.sbsp.2
where
.DELTA.IS THE AMOUNT OF DEFORMATION (IN .mu.M), AND
B.sub.1, B.sub.2 ARE CONSTANTS INDEPENDENT OF .DELTA..
Values of b.sub.1 range from 10.sup.-3 dB/m .mu.m.sup.-b.sbsp.2 for small periodicity (bend repeat distance 0.25 mm) to 10.sup.-16 for large periodicity (bend repeat distance 20 mm), and values of b.sub.2 range from 2-4 for multimode fibers to 4-6 for single mode fibers.
U.S. Pat. No. 3,602,037 discloses an apparatus for measuring minute deflections of a thin glass rod in a liquid flow gauge to determine the liquid flow or small movements acting on the rod. This known proposal for a liquid flow gauge uses absorption of a light beam injected into the rod on reflection from its walls. The rod is straight, and is provided with a light absorbing coating of higher refractive index than the rod. This liquid flow gauge uses a reflection configuration of the light passing both ways through the device, i.e., through the rod, in order to determine the deflection of the rod. This design especially increases the scattered light level making the gauge relatively insensitive and gives a non-linear relationship between the light intensity and the deflection of the rod. It operates over a limited range of deflections and it is not well adapted to use fibre optic coupling. It is difficult to measure tensile stresses of objects other than the rod itself and can only be adapted to measure bending shear stresses with a reasonable sensitivity.
It is an object of the present invention to provide an apparatus for measuring strain in solid objects, especially tensile strain, which apparatus is relatively simple in its design and which can replace standard strain gauges. This invention seeks to increase the sensitivity of an optical fibre to bending, rather than the contrary.